A group-III nitride semiconductor light-emitting device has a direct-transition-type energy band gap corresponding to the range from visible light to ultraviolet light, and has high emission efficiency. Therefore, the group-III nitride semiconductor light-emitting device has been used as a light-emitting device, such as an LED or an LD.
When the group-III nitride semiconductor light-emitting device is used for an electronic device, it is possible to obtain an electronic device having better characteristics, as compared to when a group III-V compound semiconductor according to the related art is used.
In general, a single crystal wafer made of a group-III nitride semiconductor is obtained by growing a crystal on a single crystal substrate made of a different material. There is large lattice mismatch between the substrate and a group-III nitride semiconductor crystal epitaxially grown on the substrate. For example, when a gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is 16% lattice mismatch therebetween. When a gallium nitride is grown on a SiC substrate, there is 6% lattice mismatch therebetween.
In general, the large lattice mismatch makes it difficult to epitaxially grow a crystal on the substrate directly. Even though the crystal is grown on the substrate, it is difficult to obtain a crystal having high crystallinity.
Therefore, a method has been proposed in which, when a group-III nitride semiconductor crystal is epitaxially grown on a sapphire single crystal substrate or a SiC single crystal substrate by a metal organic chemical vapor deposition (MOCVD) method, a so-called low temperature buffer layer made of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) is formed on the substrate and a group-III nitride semiconductor crystal is epitaxially grown on the buffer layer at a high temperature (for example, see Patent Documents 1 and 2).
In addition, a technique has been proposed which forms a buffer layer using methods other than the MOCVD method. For example, the buffer layer is formed on a substrate by a sputtering method, and the substrate is formed of sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, or a group-III nitride compound semiconductor single crystal. Among these materials, an a-plane sapphire substrate is preferable (for example, Patent Documents 3 and 4).
However, in the methods disclosed in Patent Documents 1 to 4, it is difficult to obtain a group-III nitride compound semiconductor having sufficiently high crystallinity.
Further, a method has been proposed which forms a buffer layer using a radio frequency sputtering method and grows a crystal having the same composition as the buffer layer using an MOCVD method (for example, Patent Document 5). However, in the method disclosed in Patent Document 5, it is difficult to form a stable and good crystal on a substrate.
Furthermore, a method has been proposed in which the initial voltage of a sputtering apparatus is lower than 110% of the sputtering voltage when a buffer layer is formed on a substrate by a sputtering method (for example, Patent Document 6). The methods disclosed in Patent Documents 3 to 6 form the buffer layer using a sputtering method without using an expensive material used in the MOCVD method.    [Patent Document 1] Japanese Patent No. 3026087    [Patent Document 2] JP-A-4-297023    [Patent Document 3] Japanese Patent No. 3440873    [Patent Document 4] Japanese Patent No. 3700492    [Patent Document 5] JP-B-5-86646    [Patent Document 6] JP-A-2001-308010